fl_wrapper: sanitise to prevent buffer overflow warnings

Signed-off-by: Alejandro Mery <amery@jpi.io>

src/tools-source/fl_wrapper/fl_wrapper.c

@@ -23,7 +23,6 @@
  *  - /usr/include/dlfcn.h
  */
 
-
 /* Headers and prototypes */
 
 #ifndef DEBUG
@@ -110,16 +109,17 @@ static void * get_dl_symbol(char * symname)
 
 static inline ssize_t readlink_from(char *buf, size_t buf_len, const char *tpl, ...)
 {
+	char pathname[buf_len];
 	ssize_t l;
 	va_list ap;
 
 	va_start(ap, tpl);
-	vsnprintf(buf, buf_len, tpl, ap);
+	vsnprintf(pathname, buf_len, tpl, ap);
 	va_end(ap);
 
-	if ((l = readlink(buf, buf, buf_len)) < 0)
-		return -1;
-	buf[l] = '\0';
+	l = readlink(pathname, buf, buf_len);
+	if (l >= 0)
+		buf[l] = '\0';
 	return l;
 }
 
@@ -301,32 +301,43 @@ static void handle_fileat_access_before(const char * func, int dirfd, const char
 	}
 }
 
-static const char *make_file_absolute(char *absfile, const char *file)
+static const char *make_file_absolute(char *buf, size_t buf_size, const char *file)
 {
-	if (file[0] == '/')
+	if (file[0] == '/') {
 		return file;
-	else {
-		char cwd[PATH_MAX];
-		if (getcwd(cwd, PATH_MAX) == NULL)
-			return NULL;
-		snprintf(absfile, PATH_MAX, "%s/%s", cwd, file);
-		return absfile;
+	} else if (getcwd(buf, buf_size)) {
+		size_t l0 = strlen(buf);
+		size_t l1 = strlen(file);
+
+		if ((l0 + l1 + 1) < buf_size) {
+			buf[l0++] = '/';
+			memcpy(buf + l0, file, l1 + 1);
+			return buf;
+		}
 	}
+
+	return NULL;
 }
 
-static const char *make_fileat_absolute(char *absfile, int dirfd, const char *file)
+static const char *make_fileat_absolute(char *buf, size_t buf_size,
+					int dirfd, const char *file)
 {
 	if (file[0] == '/')
 		return file;
 	else if (dirfd == AT_FDCWD)
-		return make_file_absolute(absfile, file);
-	else {
-		char cwd[PATH_MAX];
-		if (readlink_from(cwd, PATH_MAX, "/proc/self/fd/%d", dirfd) < 0)
-			return NULL;
-		snprintf(absfile, PATH_MAX, "%s/%s", cwd, file);
-		return absfile;
+		return make_file_absolute(buf, buf_size, file);
+	else if (readlink_from(buf, buf_size, "/proc/self/fd/%d", dirfd) > 0) {
+		size_t l0 = strlen(buf);
+		size_t l1 = strlen(file);
+
+		if ((l0 + l1 + 1) < buf_size) {
+			buf[l0++] = '/';
+			memcpy(buf + l0, file, l1 + 1);
+			return buf;
+		}
 	}
+
+	return NULL;
 }
 
 /* sort of, private realpath, mostly not readlink() */
@@ -435,7 +446,7 @@ static void handle_file_access_after(const char * func, const char * file,
 	else if (strcmp(file, rlog) == 0) return;
 	else if (lstat(file, &st) < 0) return;
 
-	file2 = make_file_absolute(absfile, file);
+	file2 = make_file_absolute(absfile, sizeof(absfile), file);
 	if (file2 == NULL) {
 		ERR("%s(\"%s\"): failed to make absolute!", func, file);
 		return;
@@ -462,7 +473,7 @@ static void handle_fileat_access_after(const char * func, int dirfd, const char
 	else if (strcmp(file, rlog) == 0) return;
 	else if (fstatat(dirfd, file, &st, AT_SYMLINK_NOFOLLOW) < 0) return;
 
-	file2 = make_fileat_absolute(absfile, dirfd, file);
+	file2 = make_fileat_absolute(absfile, sizeof(absfile), dirfd, file);
 	if (file2 == NULL) {
 		ERR("%s(%d, \"%s\"): failed to make absolute!", func, dirfd, file);
 		return;